Conventional turbojet combustion chambers comprise an inner wall, an outer wall, and in the upstream region of the chamber, an annular end wall disposed between said inner and outer walls. The chamber end wall supports injector heads that spray fuel into the combustion chamber.
Those conventional combustion chambers also have an annular fairing serving firstly to cover the upstream (i.e. front) end of said chamber end wall together with said injector heads so as to protect them from any impact (as can occur if a bird or a block of ice is ingested into the turbojet), and secondly to ensure that the chamber end wall is aerodynamically contoured allowing air to flow with little head loss.
In the present application, “upstream” and “downstream” are defined relative to the normal flow direction of gas (from upstream to downstream) through the turbomachine, and the adjectives “inner” and “outer” are used relative to a radial direction, i.e. a direction perpendicular to the axis of rotation X of the turbomachine rotor. Thus, the inner portion of an element is closer to the axis X than is the outer portion of the same element.
Certain known fairings are made up of two separate and concentric annular parts commonly referred to as “cowls”, that extend around the inner periphery and the outer periphery of the chamber end wall. These inner and outer “cowls” are fastened to the combustion chamber and they are separated by an annular gap that gives access to the injector heads and through which the fuel injectors pass that are connected to the injector heads. A “cowl” type fairing is described for example in document EP 1 265 031 A1.
Other so-called “one-piece” fairings are also known that are made from a single annular part. The two fairing “cowls” are then interconnected by bars that define between them openings through which the fuel injectors pass. In half-section in an axial plane containing the axis of rotation X, the fairing presents a shape that is substantially semicircular. Since it is more rigid, that type of fairing is better at withstanding stresses of vibratory origin than are the previously-described fairings with cowls. A one-piece fairing is described for example in document U.S. Pat. No. 6,148,600.
Fairings are generally bolted since assembly by bolting provides much greater latitude in terms of maintenance than does assembly by welding.
To mount a fairing on a chamber end wall, the inner and outer edges of the fairing are fastened by means of bolts that are regularly distributed around the chamber end wall. During this step, the bolt needs to be tightened quite considerably in order to take up assembly clearances that are inherent to fabrication and mounting tolerances, and that has the drawback of causing the fairing to lose its annular shape, the inner and/or outer edges of the fairing forming deformation lobes between pairs of bolts, giving these edges a “daisy” shape. These lobes cause gaps to appear between the assembled parts, giving rise to air leakage and head losses. In addition, given said mounting clearances, the mechanical stiffness of the assembly leads either to tightening with torque that is greater than can be accommodated by the bolt and/or the fairing, or else to insufficient contact for friction to pass operating forces via the bolted connections.
To reduce those drawbacks significantly, a known solution consists in making slots in the edges of the fairing, between the bolts, in order to provide a little more flexibility while the fairing is being put into place, and thus improve the actual clamping of the parts. Nevertheless, that solution presents other drawbacks: in operation the slots lead to leaks of air that are harmful from an aerodynamic point of view and they also run the risk of constituting crack initiation points.